The United States, with about 5 percent of the world population, consumes about 40 percent of its resources, most of which are used once and discarded. Disposal of the resulting waste material was handled, until fairly recent times, in the simplest possible manner and not looked upon as a serious environmental problem. More recently, growth in both population and consumption per capita have multiplied the production of waste whose environmentally acceptable disposal is compounded by the neglect of the past.
Waste is produced in all the forms of matter: gaseous, liquid and solid. This invention is directed toward the effective disposal of those liquid and solid wastes whose contaminants are predominantly combustible, in many cases with the recovery of valuable energy.
Solid waste is the form currently receiving the most publicity, particularly Municipal Solid Waste (MSW). According to the Environmental Protection Agency (EPA), 157.7 million tons of solid waste were discarded in 1966. Formerly dumped in a convenient open area, or barged to sea, "garbage dumps", upgraded to "landfills", are getting filled up. An April 1987 Worldwatch Institute Study estimated that by 1990 half of U.S. cities will have exhausted their landfills. Owners of landfills which still have capacity are escalating the "tipping fee" they charge for accepting solid waste. The average fee nearly doubled in 2 years, from $19.52 per ton in 1986 to $34.69 in 1988. Buried in these averages are fees as high as $100 per ton in some New England communities. At the same time, the EPA and international agreements have curtailed ocean dumping.
While environmentalists are decrying the diversion of land, the odors and polluted drainage emanating from landfills, conservationists are protesting the waste of natural resources in discarded iron, glass, aluminum and plastics, estimated by EPA to comprise, in 1986, 10.6, 11.8, 1.7, and 10.3 million tons, resp.
Under economic and environmental pressures, municipalities are increasingly turning to methods of burning (incinerating) their MSW, generally with recovery of energy. Slightly over 100 incinerators were in operation at the end of 1988, with 28 under construction and about 200 on the drawing boards. These "Waste-to Energy" projects come in two main forms: "Mass Burn" and "Resource Recovery". In the former, the MSW is burned essentially as received, in atmospheric boilers specially designed to process the often heavy and oddly shaped non-combustible items to an incinerator ash.
Resources Recovery projects, naturally favored by conservationists, process the raw MSW to recover recyclable materials (generally iron, aluminum and glass), the remainder being an improved "Refuse Derived Fuel" (RDF) which can, in many cases, be burned in existing boilers designed for coal. Since RDF has a low density and heating value, and there are other practical disadvantages to substituting it for their usual fuel, utilities are in some cases reluctant, or willing to pay only a heavily discounted price. In other cases, local powerplants are taking steps to utilize the RDF in the interest of helping to solve a common waste problem.
Mass burning projects generally face serious environmental opposition. Nearby residents fear that atmospheric emission regulations (at present limited to particulate matter) and standards of operation do not adequately protect their health. At least 26 toxic pollutants have been identified in incinerator flue gas. Moreover, the unburned residue (roughly 20-30 percent), contains hazardous substances. In EPA tests, every sample of "fly ash" (fine particles recovered from stack gases) contained unacceptable levels of toxic metals, such as lead and cadmium, and "bottom ash" contained unacceptable levels in 10-30 percent of the cases. Landfilling of hazardous ash requires very expensive precautions.
The EPA has announced plans to publish a list of incinerator air pollutants by November 1988 and propose New Source Performance Standards a year later, a regulatory effort which does not satisfy environmentalists nor congress. In the meantime, EPA is requiring new (not existing) incinerators to employ "best available technology", generally interpreted to mean scrubbers of some sort plus a particulate filter or precipitator. Congress, however, was unable to pass in 1988 either the Clean Air Act Amendment or the Resources Conservation and Recovery Act (RCRA). The impasse reflects, to a considerable degree, the difficulty and expense of removing the pollutants from the large volume of flue gas, characteristic of atmospheric pressure combustion.
Burning of RDF, on the other hand, faces comparatively little opposition, in part because it contains less sulfur than the coal for which it is substituted. However, there remains a concern for unquantified new pollutants, such as chlorine compounds, resulting from the substitution.
At the time of this application mass burning accounts for about three-fourths of the waste-to-energy projects but authorities expect the resource recovery alternative to increase its share in the future.
Although quite variable, MSW has a typical heating value of roughly 5000 Btu/Lb., less than half that of a good steam coal. Its comparatively high moisture content (20-35 percent), which must be evaporated in any atmospheric boiler, detracts seriously from the recoverable energy.
Considering its already high moisture and low heating value, it is understandable that most resource recovery units process the MSW dry, utilizing a variety of solids shredding, conveying and separating operations. Early installations encountered difficulties with stalling, clogging, equipment failure, explosions and fires. Labor and maintenance were high and availability low; recovery of recyclable components poorer than expected. With experience, resource recovery operations have improved but remain less economical and dependable than would be desired.
U.S. Pat. No. 4,624,417 (Gangi) describes a wet resource recovery process in which the separations are performed on an aqueous slurry of shredded MSW, in continuous equipment resembling that used to process wood chips to paper pulp. Equipment is said to function more dependably, without fire or explosion hazard. RDF is produced as a wet solid containing about 50 percent water and may be subsequently dried and pelletized to a saleable fuel. Drying and pelletizing adds considerably to the labor, space requirement and expense of the process.
For the atmospheric combustion of a waste to be self-sustaining, i.e., to proceed without the expense of supplemental fuel, it must have a combustible content of about 30 percent. Self-sustaining combustion is not necessarily good enough; the temperature must be sufficiently high to destroy stable pollutants, such as PCBs, furans and dioxin. With air preheat, MSW and RDF can usually attain a satisfactory temperature. To incinerate wastes of lower combustible content, including common aqueous liquid wastes, supplemental fuel is required.
Most solid and many liquid wastes are hydrophilic, i.e., have a natural affinity for water, making them difficult to dry (and keep dry). As a related characteristic, they tend to be fibrous so that considerable water is required to make a fluid (pumpable) slurry with them. "Pumpability" depends somewhat on system and type of pump. It has been variously defined as a maximum viscosity in the range of 500-1000 Centipoises. Typically, the maximum pumpable concentration is roughly 10-15 percent dry solids. Such a slurry would not burn (without supplemental fuel) in an atmospheric boiler but could be made to burn, with the recovery of some energy, under pressure. Even so, heating so much water to combustion temperature is relatively inefficient.
Patent B discloses that a slurry of a hydrophillic fuel can be concentrated by heating it to a temperature at which molecular rearrangement occurs, with evolution of heat and the splitting off of carbon dioxide and water, resulting in a less-hydrophilic and fibrous fuel for which the maximum pumpable solids concentration is considerably increased.
The University of North Dakota Research Center (UNDERC) has carried out a large number of such slurry carbonizations, which they call Hydrothermal Treatment. Most of this work has concerned high moisture fossil fuels occuring in the state, but they have reported that the "energy density" (approximately equivalent to solids concentration) of a slurry of wood fiber has been increased by 300 percent. This means that a slurry which can be pumped up to only about 10 percent concentration can be dewatered, after carbonization, to a pumpable concentration of about 40 percent.
Using EPA figures for 1986, paper and cardboard account for an average 35.6 percent and yard waste another 20.1 percent of discarded solid waste. Adding in wood, textiles, rubber, leather and food wastes, the content of fibrous components comprises about three-quarters of the total. It may therefore be expected that the concentration at which an RDF slurry can be pumped will be increased from about 10-15 to about 35-40 percent by heating to a carbonizing temperature (about 350-550 degrees F.).
Of the major liquid wastes, municipal sewage has had for many years a respectable treatment background. Established methods of biological treatment are comparatively satisfactory, but produce a by-product of sewage sludge whose existing means of disposal (ocean dumping, incineration, landfill and composting) are less acceptable. About 3 million tons per year (about 40 percent of the total) are landfilled. The last congress passed the Ocean Dumping Ban Act of 1988 which seeks to discourage disposal of sewage sludge at sea by making it more expensive, but does not prevent it nor satisfy environmentalists.
Industrial liquid wastes, whose characteristics vary over a broad spectrum are, in some cases, treated effectively and, in other cases, utilize disposal methods (including incineration, ponding, landfill and deep well injection) which are either expensive or controversial. EPA estimated that, in 1987, 60 percent of hazardous waste was being pumped into wells, including over 90 percent of that originating with the organic chemical, petroleum and steel industries. At that time there were 186 such wells in 13 states, with 70 percent of the activity in Texas and Louisiana.
In addition to the well known biological methods, some liquid industrial wastes respond well to treatment with activated carbon and to a molecular filtration known as reverse osmosis. These treatments are practically limited to dilute wastes, generally containing less than about 2 percent organic contaminant.
Solid wastes can, in general, be incinerated (although at low energy efficiency). Dilute non-toxic aqueous wastes have, for the most part, acceptable treatment methods. This leaves, however, a substantial gap encompassing toxic wastes and those with roughly 2 to 30 percent combustible content, for which the known art is relatively unsatisfactory.
Among the wastes falling in this concentration range, the best known is sewage sludge, which has a maximum pumpable solids concentration, depending upon digestion and other treatments, of roughly 10 percent. Others include aqueous wastes from conversions of low rank fossil fuels, such as peat, lignites and sub-bituminous coal, into more marketable higher heating value chars (or the like).
In addition to incineration, patent and technical literature describe two other methods of purifying aqueous wastes by burning combustible impurities from them. Both employ elevated pressures as well as temperatures. The earlier of these methods, known as Wet Air Oxidation (WAO), is licensed by Zimpro, Inc. The second, known as Supercritical Water Oxidation (SCWO), is licensed by Modar, Inc. They are more thermally efficient than incineration because the purified water is discharged in liquid state, conserving its latent heat. Consequently, the combustible concentration at which they can be self-sufficient in energy is much lower than with atmospheric combustions. Also, the flue gas is ordinarily free of pollutants and requires little or no treatment.
WAO oxidizes combustible impurities from aqueous wastes, imposing sufficient pressure to maintain them in liquid phase at temperatures which generally range from 400-650 F. SCWO oxididizes the impurities at temperatures and pressures above water's critical of 705.4 F. and 3200 psi. While both methods are effective, they require expensive equipment, suitable for pressures generally in the range of 2000-4000 psi.